The present invention relates to a projection exposure apparatus for using a circular or elliptical mask on which a rectangular pattern area is formed.
A projection exposure apparatus is used for transferring a circuit pattern of a semiconductor integrated circuit or a liquid crystal substrate onto a photosensitive substrate under a lithography technique. Such projection exposure apparatuses are generally classified based on their exposure systems into two known types, namely a mirror projection aligner and a reduction projection exposure apparatus called xe2x80x9cstepperxe2x80x9d. In the mirror projection aligner, a mask and a wafer are simultaneously scanned with respect to the projection optical system interposed therebetween such that a pattern on the mask is transferred onto the wafer with equivalent magnification. FIG. 10A shows a typical example of the mask used in the mirror projection aligner. The aligner transfers an entire pattern on the mask onto the wafer by one time of scanning. Accordingly, the pattern having the same dimension as that of the wafer has been also formed on the mask. Since the wafer generally has a circular shape, exposure shot areas are designed so as to form as many semiconducter chips as possible on a wafer surface without wasting wafer area. Therefore, the mask has had the pattern as shown in FIG. 10A, and a contour of the mask has been also circular in conformity with the wafer. Such a mask has been secured to a mask stage by a vacuum chuck or the like by using portions 52A to 52D on which no pattern is formed.
On the other hand, the stepper is an apparatus for trasferring a reticle (mask) image onto a wafer by projection through a reduction projection optical system while reducing the image. In recent years, although the pattern for a circuit becomes finer and finer, it is difficult to obtain sufficient resolution for the finer pattern by the mirror projection system described above. For this reason, the reduction projection exposure system using the stepper becomes dominant. The stepper successively exposes a plurality of chips (shot areas) into which a wafer is divided. The stepper is classified to two systems, namely the step-and-repeat system in which only a wafer is step-driven with respect to a projection optical system while a reticle is fixed, and the step-and-scan system in which a reticle and a wafer are relatively scanned with respect to a projection optical system. FIG. 10B shows a plan view of a reticle used for such reduction projection exposure systems. The reticle 53 has a rectangular or square pattern section 54 which has a shape equivalent to the shape of chips formed on the wafer. In general, the pattern section 54 is covered with a pellicle for preventing a pattern from adhesion of contaminants. A pellicle frame 55 for supporting the pellicle is provided outside the pattern section 54. Attracting sections 56A to 56D for securing the reticle 53 to a reticle stage are provided at four corners outside the pellicle frame 55 so that they extend in a scanning direction for the reticle.
In any type of the projection exposure apparatuses described above, the contour of the reticle (mask) has been determined in conformity with a shape of an exposure field usable for exposure at one time. Namely, a circular mask has been used for the projection exposure apparatus of the mirror projection system, while a rectangular reticle has been used for the reduction type projection exposure apparatus of. However, as the development of fine patterns and the development of large chips are advancing in recent years, the rectangular or square shape of the reticle, which has been used in the reduction projection exposure apparatus, gives rise to an inconvenience from the viewpoint of rigidity and processing accuracy. In general, the reticle is made of a material of quartz glass from which a circular reticle would be produced easier. Therefore, it is advantageous for reticle production to produce reticles having circular contours. If the reticle has a circular shape, such a reticle is not subject to partial damage, because the flexure against external force or the like is liable to act isotropically on a plane of such a reticle. Accordingly, it is desired to use a circular reticle also in the reduction projection exposure apparatus. However, there has been hitherto no case in which any circular reticle is used in the reduction type projection exposure apparatus. Various problems must be solved upon the use of a circular reticle. For example, installation of a circular reticle to a reticle stage would require accurate positional adjustment, especially adjustment for arrangement in a rotational direction (orientation). In addition, it would be also necessary to investigate arrangement of a pattern and attracting sections in a circular reticle, installation of a pellicle, and positions for attracting the reticle on a reticle stage in conformity therewith.
An object of the present invention is to provide a novel projection exposure apparatus including a mechanism for performing positional adjustment for a circular reticle on a reticle stage efficiently and highly accurately.
Another object of the present invention is to provide a projection exposure apparatus including a prealignment stage which is capable of previously adjusting a rotational error before arranging a circular reticle on a reticle stage.
Still another object of the present invention is to provide a method which makes it possible to efficiently perform various calibration processes such as adjustment for image formation characteristics of a projection optical system, and positional adjustment for a reticle stage and a photosensitive substrate stage during exchange of a photosensitive substrate in a series of exposure operations for the photosensitive substrate by using a projection exposure apparatus of the step-and-scan system.
Still another object of the present invention is to provide a projection exposure method comprising a step which makes it possible to efficiently perform various calibration processes such as adjustment for image formation characteristics of a projection optical system, and positional adjustment for a reticle stage and a photosensitive substrate stage during exchange of a photosensitive substrate in a series of exposure operations for the photosensitive substrate by using a projection exposure apparatus of the step-and-scan system.
According to a first aspect of the present invention, there is provided a projection exposure apparatus for exposing a photosensitive substrate with a pattern formed on a circular mask having a rectangular pattern area by projection through a projection optical system, comprising:
an illumination optical system for illuminating the circular mask;
a mask stage;
a stage for moving the photosensitive substrate two-dimensionally;
a prealignment stage for pre-aligning the circular mask for its orientation, the prealignment stage comprising a detecting means for detecting an orientational error of the circular mask from a predetermined orientation on the prealignment stage, a rotatable means for rotating the circular mask on the prealignment stage, and a control means for controlling the rotatable means on the basis of the orientational error so that the circular mask has the predetermined orientation; and
a transport means for transporting the circular mask from the prealignment stage to the mask stage while maintaining a pre-aligned orientation of the circular mask. The detecting means may comprise two optical sensors for detecting marks formed at least at two positions on the circular mask respectively, a moving unit for relatively moving the prealignment stage with respect to the optical sensors so that each of the marks on the circular mask relatively passes across each of optical paths of the optical sensors, and a calculating means for calculating the orientational error on the basis of a difference between detected positions of the marks detected by the two optical sensors, and the rotatable means may be composed of a rotatable stage installed on the prealignment stage.
According to the first aspect of the present invention, the two marks for positional adjustment affixed to the circular reticle (circular mask) are detected by moving the prealignment stage for the circular reticle with respect to the detecting means comprising detecting devices such as the optical sensor to determine the orientational (rotational) error xcex94xcex8 from the predetermined orientation of the circular reticle on the basis of the difference between detected positions of the two marks. The orientation of the circular reticle can be pre-aligned by rotating the circular reticle by the error xcex94xcex8 in a rotational direction for correction on a rotatable stage installed on a prealignment stage 11 as shown in FIGS. 3A and 3B. The rotatable stage is used separately from the reticle stage (mask stage). Accordingly, it is possible to correct a rotational error which is in an amount incapable of xcex8-correction on the reticle stage. It is unnecessary to place the reticle on the reticle again even when the rotational error is large.
According to a second aspect of the present invention, there is provided a projection exposure apparatus for exposing a photosensitive substrate with a pattern formed on a circular mask having a rectangular pattern area and having a cutout by projection through a projection optical system, comprising:
an illumination optical system for illuminating the circular mask;
a mask stage;
a stage for moving the photosensitive substrate two-dimensionally;
a prealignment stage for pre-aligning the circular mask, the prealignment stage comprising two fastening sections for fastening the cutout of the circular mask, and a movable pin for moving the circular mask on the prealignment stage; and
a transport means for transporting the pre-aligned circular mask from the prealignment stage to the mask stage.
According to the second aspect of the present invention, when the circular reticle having the cutout (orientation flat) is used, the cutout is fastened at two positions on the prealignment stage of the present invention as shown in FIG. 2. Therefore, the orientation of the circular reticle can be easily pre-aligned.
According to a third aspect of the present invention, there is provided a projection exposure apparatus for exposing a photosensitive substrate with a pattern formed on a circular mask having a rectangular pattern area and having a cutout by projection through a projection optical system, comprising:
an illumination optical system for illuminating the circular mask;
a mask stage;
a stage for moving the photosensitive substrate two-dimensionally; and
a transport means for transporting the circular mask to the mask stage while maintaining a pre-aligned orientation of the circular mask.
In the third aspect, the transport means preferably comprises a transport arm having at least three pawls for grasping the circular mask for performing prealignment for the circular mask by fastening the cutout of the circular mask with two pawls of the at least three pawls, a rotatable arm having an attracting section for attracting the circular mask for transporting the circular mask to the mask stage by performing rotary movement, and a transport control unit for controlling operations of the transport arm and the rotatable arm. The circular reticle (circular mask) can be pre-aligned while transporting the circular reticle, since the cutout of the circular reticle is fastened at two positions by the pawls of the transport unit (rotatable arm) for transporting the circular reticle to the reticle stage (mask stage). Since the circular reticle is pre-aligned as described above, it is unnecessary to provide a complex mechanism for adjusting the orientation of the circular reticle on the reticle stage.
In the projection exposure apparatus of the present invention, the mask stage preferably comprises an attracting section for holding the circular mask at a position outside each side of a rectangle which defines the pattern area in the circular mask. Since the attracting section is provided at such a position, the influence of acceleration in a scanning direction is hardly exerted in an apparatus of the step-and-scan system, and the position of the reticle is not deviated during scanning. The projection exposure apparatus is preferably a reduction projection exposure apparatus.
According to a fourth aspect of the present invention, there is provided a method for performing calibration for a projection optical system included in a projection exposure apparatus used for exposing a photosensitive substrate by projection with a pattern formed on a mask having a teg mark (teg: top edge gilt, t.e.g.), comprising the steps of:
arranging a sensor at an exposure position for the photosensitive substrate;
arranging the mask at a position at which the sensor and the teg mark on the mask are in a conjugate relation through the projection optical system; and
detecting an image of the teg mark formed through the projection optical system by using the sensor while relatively moving the sensor and the mask,
wherein the calibration being performed after completion of an exposure operation for the photosensitive substrate and during a period in which the photosensitive substrate is exchanged by withdrawing the photosensitive substrate from an exposure field of the projection optical system.
According to a fifth aspect of the present invention, there is provided a projection exposure method of the step-and-scan system for exposing a photosensitive substrate by projection with a pattern formed on a mask having a teg mark by synchronously scanning the mask and the photosensitive substrate through a projection optical system, comprising the steps of:
exposing the photosensitive substrate by synchronously scanning the mask and the photosensitive substrate through the projection optical system while illuminating the mask;
exchanging the photosensitive substrate by withdrawing the photosensitive substrate from an exposure field of the projection optical system;
arranging a sensor at an exposure position for the photosensitive substrate, and arranging the mask at a position at which the sensor and the teg mark on the mask are in a conjugate relation through the projection optical system, during the step of exchanging the photosensitive substrate; and
performing calibration for the projection optical system by detecting an image of the teg mark formed through the projection optical system by using the sensor.
According to the fourth and fifth aspects of the present invention, the teg mark affixed to the reticle (mask) can be measured by using the sensor arranged in juxtaposition with a wafer stage through the projection optical system upon exchange of the wafer in the projection exposure process. Accordingly, the calibration for, for example, image formation characteristics of the projection optical system can be performed without any decrease in throughput. The methods of the present invention are preferably applied to a projection exposure method for using a circular mask. The calibration can be performed for at least one adjustment process with respect to image formation characteristics of the projection optical system, and positional adjustment for the mask and the photosensitive substrate.
The term xe2x80x9cmaskxe2x80x9d used herein is a concept including the reticle as well. In this specification, the term xe2x80x9ccircular reticlexe2x80x9d is a concept including not only the circular reticle but also elliptical and regular polygonal reticles, and the term xe2x80x9crectangular pattern areaxe2x80x9d is a concept including square and oblong pattern areas. The term xe2x80x9ctransport meansxe2x80x9d corresponds to a xe2x80x9ctransport mechanismxe2x80x9d in embodiments described below.